Steganography (i/ˌstɛɡ.əˈnɒɡ.rə.fi/, STEG-ə-NOG-rəfee) is the practice of concealing a file, message, image, or video within another file, message, image, or video. The word steganography combines the Greek words steganos (στεγανός), meaning "covered, concealed, or protected", and graphein (γράφειν) meaning "writing".

The first recorded use of the term was in 1499 by Johannes Trithemius in his Steganographia, a treatise on cryptography and steganography, disguised as a book on magic. Generally, the hidden messages appear to be (or be part of) something else: images, articles, shopping lists, or some other cover text. For example, the hidden message may be in invisible ink between the visible lines of a private letter. Some implementations of steganography that lack a shared secret are forms of security through obscurity, whereas key-dependent steganographic schemes adhere to Kerckhoffs's principle.[1]

The advantage of steganography over cryptography alone is that the intended secret message does not attract attention to itself as an object of scrutiny. Plainly visible encrypted messages—no matter how unbreakable—arouse interest, and may in themselves be incriminating in countries where encryption is illegal.[2] Thus, whereas cryptography is the practice of protecting the contents of a message alone, steganography is concerned with concealing the fact that a secret message is being sent, as well as concealing the contents of the message.

Steganography includes the concealment of information within computer files. In digital steganography, electronic communications may include steganographic coding inside of a transport layer, such as a document file, image file, program or protocol. Media files are ideal for steganographic transmission because of their large size. For example, a sender might start with an innocuous image file and adjust the color of every 100th pixel to correspond to a letter in the alphabet, a change so subtle that someone not specifically looking for it is unlikely to notice it.

The first recorded uses of steganography can be traced back to 440 BC when Herodotus mentions two examples in his Histories.[3]Histiaeus sent a message to his vassal, Aristagoras, by shaving the head of his most trusted servant, "marking" the message onto his scalp, then sending him on his way once his hair had regrown, with the instruction, “When thou art come to Miletus, bid Aristagoras shave thy head, and look thereon." Additionally, Demaratus sent a warning about a forthcoming attack to Greece by writing it directly on the wooden backing of a wax tablet before applying its beeswax surface. Wax tablets were in common use then as reusable writing surfaces, sometimes used for shorthand.

In his work PolygraphiaeJohannes Trithemius developed his so-called "Ave-Maria-Cipher" that can hide information in a Latin praise of God. "Auctor Sapientissimus Conseruans Angelica Deferat Nobis Charitas Potentissimi Creatoris" for example contains the concealed word VICIPEDIA.[4]

Steganography has been widely used, including in recent historical times and the present day. Known examples include:

Hidden messages within wax tablet—in ancient Greece, people wrote messages on wood and covered it with wax that bore an innocent covering message.

Hidden messages on messenger's body—also used in ancient Greece. Herodotus tells the story of a message tattooed on the shaved head of a slave of Histiaeus, hidden by the hair that afterwards grew over it, and exposed by shaving the head. The message allegedly carried a warning to Greece about Persian invasion plans. This method has obvious drawbacks, such as delayed transmission while waiting for the slave's hair to grow, and restrictions on the number and size of messages that can be encoded on one person's scalp.

Hidden messages on paper written in secret inks, under other messages or on the blank parts of other messages

Messages written in Morse code on yarn and then knitted into a piece of clothing worn by a courier.

In the early days of the printing press, it was common to mix different typefaces on a printed page due to the printer not having enough copies of some letters in one typeface. Because of this, a message could be hidden using two (or more) different typefaces, such as normal or italic.

During and after World War II, espionage agents used photographically produced microdots to send information back and forth. Microdots were typically minute (less than the size of the period produced by a typewriter). World War II microdots were embedded in the paper and covered with an adhesive, such as collodion. This was reflective, and thus detectable by viewing against glancing light. Alternative techniques included inserting microdots into slits cut into the edge of post cards.

During WWII, Velvalee Dickinson, a spy for Japan in New York City, sent information to accommodation addresses in neutral South America. She was a dealer in dolls, and her letters discussed the quantity and type of doll to ship. The stegotext was the doll orders, while the concealed "plaintext" was itself encoded and gave information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman.

Jeremiah Denton repeatedly blinked his eyes in Morse Code during the 1966 televised press conference that he was forced into as an American POW by his North Vietnamese captors, spelling out "T-O-R-T-U-R-E". This confirmed for the first time to the U.S. Military (naval intelligence) and Americans that the North Vietnamese were torturing American POWs.

Cold War counter-propaganda. In 1968, crew members of the USS Pueblo intelligence ship held as prisoners by North Korea, communicated in sign language during staged photo opportunities, informing the United States they were not defectors, but captives of the North Koreans. In other photos presented to the US, crew members gave "the finger" to the unsuspecting North Koreans, in an attempt to discredit photos that showed them smiling and comfortable.

Image of a tree with a steganographically hidden image. The hidden image is revealed by removing all but the two least significant bits of each color component and a subsequent normalization. The hidden image is shown below.

Image of a cat extracted from the tree image above.

Modern steganography entered the world in 1985 with the advent of personal computers being applied to classical steganography problems.[5] Development following that was very slow, but has since taken off, going by the large number of steganography software available:

Concealing messages within the lowest bits of noisy images or sound files.

Concealing data within encrypted data or within random data. The message to conceal is encrypted, then used to overwrite part of a much larger block of encrypted data or a block of random data (an unbreakable cipher like the one-time pad generates ciphertexts that look perfectly random without the private key).

Pictures embedded in video material (optionally played at slower or faster speed).

Injecting imperceptible delays to packets sent over the network from the keyboard. Delays in keypresses in some applications (telnet or remote desktop software) can mean a delay in packets, and the delays in the packets can be used to encode data.

Changing the order of elements in a set.

Content-Aware Steganography hides information in the semantics a human user assigns to a datagram. These systems offer security against a nonhuman adversary/warden.

Blog-Steganography. Messages are fractionalized and the (encrypted) pieces are added as comments of orphaned web-logs (or pin boards on social network platforms). In this case the selection of blogs is the symmetric key that sender and recipient are using; the carrier of the hidden message is the whole blogosphere.

Making text the same color as the background in word processor documents, e-mails, and forum posts.

Using Unicode characters that look like the standard ASCII character set. On most systems, there is no visual difference from ordinary text. Some systems may display the fonts differently, and the extra information would then be easily spotted, of course.

Using hidden (control) characters, and redundant use of markup (e.g., empty bold, underline or italics) to embed information within HTML, which is visible by examining the document source. HTML pages can contain code for extra blank spaces and tabs at the end of lines, and colours, fonts and sizes, which are not visible when displayed.

Using non-printing Unicode characters Zero-Width Joiner (ZWJ) and Zero-Width Non-Joiner (ZWNJ).[8] These characters are used for joining and disjoining letters in Arabic and Persian, but can be used in Roman alphabets for hiding information because they have no meaning in Roman alphabets: because they are "zero-width" they are not displayed. ZWJ and ZWNJ can represent "1" and "0".

In communities with social or government taboos or censorship, people use cultural steganography—hiding messages in idiom, pop culture references, and other messages they share publicly and assume are monitored. This relies on social context to make the underlying messages visible only to certain readers.[9][10] Examples include:

Hiding a message in the title and context of a shared video or image

Misspelling names or words that are popular in the media in a given week, to suggest an alternate meaning

Hiding a Picture which can be traced by using Paint or any other Drawing tool.

All information hiding techniques that may be used to exchange steganograms in telecommunication networks can be classified under the general term of network steganography. This nomenclature was originally introduced by Krzysztof Szczypiorski in 2003.[11] In 2016, a first network steganography covering book was published by Mazurczyk et al.[12] However, network information hiding was already applied in the late 1980s by Girling[13] and Wolf.[14] Contrary to typical steganographic methods that use digital media (images, audio and video files) to hide data, network steganography uses communication protocols' control elements and their intrinsic functionality. As a result, such methods can be harder to detect and eliminate.[15]

Typical network steganography methods involve modification of the properties of a single network protocol. Such modification can be applied to the PDU (Protocol Data Unit),[16][17][18] to the time relations between the exchanged PDUs,[19] or both (hybrid methods).[20]

Moreover, it is feasible to utilize the relation between two or more different network protocols to enable secret communication. These applications fall under the term inter-protocol steganography.[21] Alternatively, multiple network protocols can be used simultaneously to transfer hidden information and so-called control protocols can be embedded into steganographic communications to extend their capabilities, e.g. to allow dynamic overlay routing or the switching of utilized hiding methods and network protocols.[22][23]

Network steganography covers a broad spectrum of techniques, which include, among others:

Steganophony — the concealment of messages in Voice-over-IP conversations, e.g. the employment of delayed or corrupted packets that would normally be ignored by the receiver (this method is called LACK — Lost Audio Packets Steganography), or, alternatively, hiding information in unused header fields.[24]

WLAN Steganography – transmission of steganograms in Wireless Local Area Networks. A practical example of WLAN Steganography is the HICCUPS system (Hidden Communication System for Corrupted Networks)[25]

Digital steganography output may be in the form of printed documents. A message, the plaintext, may be first encrypted by traditional means, producing a ciphertext. Then, an innocuous covertext is modified in some way so as to contain the ciphertext, resulting in the stegotext. For example, the letter size, spacing, typeface, or other characteristics of a covertext can be manipulated to carry the hidden message. Only a recipient who knows the technique used can recover the message and then decrypt it. Francis Bacon developed Bacon's cipher as such a technique.

The ciphertext produced by most digital steganography methods, however, is not printable. Traditional digital methods rely on perturbing noise in the channel file to hide the message, as such, the channel file must be transmitted to the recipient with no additional noise from the transmission. Printing introduces much noise in the ciphertext, generally rendering the message unrecoverable. There are techniques that address this limitation, one notable example is ASCII Art Steganography.[26]

Although no classic steganography, some types of modern color laser printers integrate the model, serial number and timestamps on each printout for traceability reasons using a dot-matrix code made of small, yellow dots not recognizable by the bare eye - see printer steganography for details.

The art of concealing data in a puzzle can take advantage of the degrees of freedom in stating the puzzle, using the starting information to encode a key within the puzzle / puzzle image.

For instance, steganography using sudoku puzzles has as many keys as there are possible solutions of a sudoku puzzle, which is 7021670999999999999♠6.71×1021. This is equivalent to around 70 bits, making it much stronger than the DES method, which uses a 56 bit key.[27]

Discussions of steganography generally use terminology analogous to (and consistent with) conventional radio and communications technology. However, some terms show up in software specifically, and are easily confused. These are most relevant to digital steganographic systems.

The payload is the data covertly communicated. The carrier is the signal, stream, or data file that hides the payload—which differs from the channel (which typically means the type of input, such as a JPEG image). The resulting signal, stream, or data file with the encoded payload is sometimes called the package, stego file, or covert message. The percentage of bytes, samples, or other signal elements modified to encode the payload is called the encoding density, and is typically expressed as a number between 0 and 1.

In a set of files, those files considered likely to contain a payload are suspects. A suspect identified through some type of statistical analysis might be referred to as a candidate.

Detecting physical steganography requires careful physical examination—including the use of magnification, developer chemicals and ultraviolet light. It is a time-consuming process with obvious resource implications, even in countries that employ large numbers of people to spy on their fellow nationals. However, it is feasible to screen mail of certain suspected individuals or institutions, such as prisons or prisoner-of-war (POW) camps.

During World War II, prisoner of war camps gave prisoners specially treated paper that would reveal invisible ink. An article in the 24 June 1948 issue of Paper Trade Journal by the Technical Director of the United States Government Printing Office, Morris S. Kantrowitz, describes, in general terms, the development of this paper. They used three prototype papers named Sensicoat, Anilith, and Coatalith. These were for the manufacture of post cards and stationery provided to German prisoners of war in the US and Canada. If POWs tried to write a hidden message, the special paper rendered it visible. The U.S. granted at least two patents related to this technology—one to Kantrowitz, U.S. Patent 2,515,232, "Water-Detecting paper and Water-Detecting Coating Composition Therefor," patented 18 July 1950, and an earlier one, "Moisture-Sensitive Paper and the Manufacture Thereof", U.S. Patent 2,445,586, patented 20 July 1948. A similar strategy is to issue prisoners with writing paper ruled with a water-soluble ink that runs in contact with water-based invisible ink.

In computing, steganographically encoded package detection is called steganalysis. The simplest method to detect modified files, however, is to compare them to known originals. For example, to detect information being moved through the graphics on a website, an analyst can maintain known clean-copies of these materials and compare them against the current contents of the site. The differences, assuming the carrier is the same, comprise the payload. In general, using extremely high compression rates makes steganography difficult, but not impossible. Compression errors provide a hiding place for data, but high compression reduces the amount of data available to hold the payload, raising the encoding density, which facilitates easier detection (in extreme cases, even by casual observation).

Some modern computer printers use steganography, including HP and Xerox brand color laser printers. These printers add tiny yellow dots to each page. The barely-visible dots contain encoded printer serial numbers and date and time stamps.[28]

The larger the cover message (in binary data, the number of bits) relative to the hidden message, the easier it is to hide the latter. For this reason, digital pictures (which contain large amounts of data) are used to hide messages on the Internet and on other communication media. It is not clear how common this actually is. For example: a 24-bit bitmap uses 8 bits to represent each of the three color values (red, green, and blue) at each pixel. The blue alone has 28 different levels of blue intensity. The difference between 11111111 and 11111110 in the value for blue intensity is likely to be undetectable by the human eye. Therefore, the least significant bit can be used more or less undetectably for something else other than color information. If this is repeated for the green and the red elements of each pixel as well, it is possible to encode one letter of ASCII text for every three pixels.

Stated somewhat more formally, the objective for making steganographic encoding difficult to detect is to ensure that the changes to the carrier (the original signal) due to the injection of the payload (the signal to covertly embed) are visually (and ideally, statistically) negligible; that is to say, the changes are indistinguishable from the noise floor of the carrier. Any medium can be a carrier, but media with a large amount of redundant or compressible information are better suited.

There are distributed steganography methods,[30] including methodologies that distribute the payload through multiple carrier files in diverse locations to make detection more difficult. For example, U.S. Patent 8,527,779 by cryptographer William Easttom (Chuck Easttom).